The present invention relates to a control device for a brushless motor used in OA apparatus, etc. such as a printer, PPC, etc.
Recently OA apparatuses such as printers, PPC's etc. have been divided into two extreme groups, one of them being of popular type pursuing a low price, while the other is of high grade type laying stress on performance. In particular, for high grade type apparatuses, it is desired to improve further performance such as increasing speed, improving in printing quality, reducing of noise, etc. Therefore high performance such as high speed, high controllability, low vibration, low noise, etc. is required for motors mounted on these high grade type apparatuses.
Among these improvements in performance, e.g. if it is tried to increase the speed, the current driving the motor is generally increased. When the current driving the motor is increased, power loss in a motor controlling circuit is also increased, which necessitates a heat evacuating structure. Therefore, there is concern that the motor and the control system therefor will become overly large.
Consequently a control device of low power loss is required for the motor. Heretofore, as example of such a control device, is shown in FIG. 6.
FIG. 6 is a diagram showing the construction of a prior art control device for a brushless motor.
In FIG. 6, reference numerals 101, 102 and 103 are motor driving coils and a capacitor 104, 105 or 106 is connected with one end of each of these motor driving coils 101, 102 and 103, while the other ends thereof are connected in common.
110 and 120 indicate driving transistor groups, which consist of transistors 111, 112 and 113, whose emitters are connected in common and grounded through a resistor 109, and transistors 121, 122 and 123, whose collectors are connected in common, respectively.
The emitters of the transistors 121, 122 and 123 are connected with the collectors of transistors 111, 112 and 113, respectively, and also with the terminals of the driving coils 101, 102 and 103.
107 is a commutation control section, which is constructed so as to output conduction switching signals to the driving transistor groups 110 and 120 so that conduction timing for the driving coils 101, 102 and 103 is optimum with respect to the position of the rotor of the motor.
108 is a speed control section, the output of which is connected with the inverted input terminal of a comparator 130. 140 is an oscillator, which is connected with the non-inverted input terminal of the comparator 130. 150 is a motor driving power supplying section, which is disposed between a main power supply 170 and the collectors of the driving transistors 120, which are connected in common. The output of the comparator 130 is inputted to the motor driving power supplying section 150 so as to control the output voltage thereof.
Operation of the prior art control device for a brushless motor constructed as described above will be explained below.
In FIG. 6, the commutation control section 107 outputs the conduction switching signals to the driving transistor groups 110 and 120 so that conduction timing for the driving coils 101, 102 and 103 is optimum with respect to the position of the rotor of the motor, and in this way the motor is driven with a high efficiency.
On the other hand, the speed control section 108 outputs control signals for controlling the motor so as to rotate it with a constant speed and outputs thereof are compared with triangular oscillation outputs outputted by the oscillator 140 in the comparator 130. Consequently, PWM (pulse width modulation) signals having pulse widths corresponding to the control signal output of the speed control section 108 are outputted by the comparator 130.
A transistor 151 constituting the motor driving power supplying section 150 ON/OFF-switches the main power supply 170 according to the PWM signal output of the comparator 130 and ON/OFF-switching signals thereof are smoothed by an inductance 153 and a capacitor 154.
That is, the output voltage of the motor driving power supplying section 150 is controlled by the PWM signal output of the comparator 130 and as a result the output voltage of the motor driving power supplying section 150 is controlled by the control signal output of the speed control section 108.
Consequently, electric power supplied to the motor driving coils 101, 102 and 103 is controlled so that the motor is controlled so as to be rotated with a predetermined speed.